Interface and strain engineering of thickness-dependent band gap widening in Mg doped ZnO nanofibrous thin films for photovoltaic applications

This study investigates how Mg incorporation and controlled variation in film thickness govern interfacial strain, lattice distortion, and defect redistribution in ZnO fibrous thin films. Films with 2–10 layers were synthesized via a layer-by-layer sol–gel approach, enabling direct assessment of thickness-dependent microstructural relaxation and surface/interface reorganization. Mg substitution reduces microstrain and dislocation density, while increased thickness promotes grain growth and strain relaxation, enhancing charge transport. Optical measurements show sharp ultraviolet absorption edges (375–378 nm) and tunable band gaps of 4.97–5.15 eV, reflecting the interplay of lattice modulation and interfacial compressive strain. Photoluminescence reveals enhanced near-band-edge emission with suppressed defect-related visible bands. Electrical characterization demonstrates thickness-dependent improvements in carrier concentration (9.6 × 10¹³–5.72 × 10¹⁴ cm⁻³), mobility (7.2–10.9 cm² V⁻¹ s⁻¹), and conductivity (1.06 × 10⁻⁴–7.26 × 10⁻⁴ S cm⁻¹). Spin-polarized DFT calculations show that atomistic strain fields and interfacial geometries modulate orbital hybridization, electronic density of states, and carrier localization, providing a microscopic explanation for the observed optical and electronic trends. These findings establish thickness-directed interface and strain engineering as a controllable route to tailor wide-bandgap Mg-doped ZnO for ultraviolet optoelectronic and transparent electronic applications.